1. Field of the Invention
The present invention relates to an optical disc which is subjected to a sampled servo type tracking and its driving apparatus, particularly, it relates to improvements of the pit configuration for sampled servo preformatted in the optical disc.
2. Description of the Prior Art
FIG. 1 shows a track sector format of a conventional optical disc described in SPIE, vol. 695, Optical Mass Data Storage 2 (1986), Page 112. In the figure, (90) indicates a sector structure per one round of track comprising 32 sectors (#0 to #31). (91) shows a block structure per one sector comprising 43 blocks (B1 to B43). Each block is consisting of a 2-byte servo field and a successive 16-byte data field and being divided into 32.times.43=1376 blocks per one track. FIG. 2 shows a pit pattern of the servo field. Pits (92), (94) and (93), (94) are slightly deviated respectively in the opposite direction relative to axes of the track centers (97), (98). Tracking sensor signal can be obtained only from these pairs of pits (called a pair of wobbled pits). Such a servo system is called a sampled servo type whose operation principle is described, for example, in SPIE, Vol. 529, Third International Conference on Optical Mass Data Storage (1985), Page 140, so that it will be omitted here. In such prior art optical disc system, since the tracking sensor signal can be obtained only from the pair of pits in the servo field, guide grooves for tracking are not necessary. Accordingly, in order to access quickly from the present track to a certain object track, as shown in FIG. 2, the servo field structures A, B are arranged alternately at every 16 tracks so that the track quantity passed during the high speed access can be counted. In FIG. 2, the track number is given as follows, EQU track number=I+(N-1).times.16
where, I=1, 2, 3 . . . 16. In the servo field structure A, N=1,3,5, . . . , and in the servo field structure B, N=2,4,6, . . . . In the servo field structures A and B, the position of one pit (92) of a pair of pits is shifted toward the track from the other pit (93). When accessing as crossing the track diagonally, the track quantity crossed can be obtained by detecting the positions of pits as illustrated in FIG. 3. In the figure, (71) indicates track centers which are present in a number at 1.5 .mu.m intervals. (72) denotes the position of the servo field which is, as shown on the right hand side of the figure, constructed as A, B for every 16 tracks. (73) generally represents a locus of an optical spot at high-speed accessing. A black spot indicated at (74) shows the intersection of the optical spot and the servo field. The servo field structure can be recognized by the black spot (74). (99) denotes a recognized signal wave form, in which an "H" level represents the servo field structure A and an "L" level represents the servo field structure B. It is to be noted that 16 tracks are counted at every change of state of the signal wave form (99), from which the number of tracks crossed during accessing can be counted and the object track can be reached immediately.
In the prior art aforementioned, through it is possible to count the tracks when an optical head is accessed at high speed, as it is clear from FIG. 3, there is such a disadvantage that it can not be detected that whether the optical spot is processing externally or internally with respect to the disc track. As a method for accessing the optical head at high speed, there is the method of taking out the speed detecting signal during accessing from the disc to control the speed of the optical head. This speed control method, when compared to the conventional method in which a glass scale is provided at the outer portion to control the speed thereby, has such advantages as eliminating the glass scale, reducing the unit size and moderating the machine accuracy. However, when using the conventional optical disc and employing this speed control method, it will be a fatal defect that the direction can not be detected. It is because that, since the seek direction of the optical head which may reverse during the speed control can not be detected, the control loop makes a positive feedback causing the optical head to runaway and collide with an inner or outer stopper to break.
In the aforesaid prior art, through the tracks can be counted up to the high seek speed of 16.times.track pitch (1.5 .mu.m)/block period (1/30.times.1/1376 sec.)=1.0 m/sec. at disc revolutions of 1800 r.p.m., as the servo field structure is changed at every 16 tracks, on the other hand, a fine count under 16 tracks is not possible. Therefore, when the remained track quantity approaches to 16, the other low speed track count technique must be used, greatly hindering the reduction of access time due to the low speed. The low speed track count technique as referred to herein is a method for counting from the number of tracks crossed by the tracking sensor signal of the sampled servo as the maximum detecting limit speed of track pitch/block period=61.9 mm/sec.
Furthermore, when controlling the speed by taking out the speed detecting signal from the disc during accessing, since the speed signal can be detected only after moving by 16 tracks, and idle time of a speed detector is lengthened and a speed control system becomes unstable, making the wide-band high-speed speed control impossible.